1. Field
The disclosure relates to a method and apparatus for fractionating a lignocellulose-based biomass.
2. Description of the Related Art
With globally increasing concern about exhaustion of resources and pollution of the environment by overuse of fossil fuels, new and renewable substitute energy resources for stably and continuously producing energy are being considered. In the ongoing development of such substitute energy resources, a technique of producing alcohol from biomass is receiving considerable attention.
The most abundant and fully renewable biomass on the planet is lignocellulose. Lignocellulose is a complex structure of a non-biodegradable aromatic polymer such as lignin, and carbohydrates such as cellulose and hemicellulose. The lignocellulose has often been used to refer to biomass. Various water-soluble fuels such as alcohol, diesel, and hydrogen, which are produced from biomass, are generally called bioenergy.
Cellulose, a significant component of lignocellulose, is a stable polysaccharide consisting of a linear chain of glucose units joined by β-1,4 glycosidic bonds, making it far more physically and chemically robust than a helical amylose consisting of glucose units joined by α-1,4 glycosidic bonds in the natural state.
Hemicellulose, another significant component of lignocellulose, is a polysaccharide with a lower degree of polymerization than cellulose. Hemicellulose consists of a polymer of pentose such as xylose as a main component and lesser amounts of a polymer of pentose such as arabinose and a polymer of hexose such as mannose, galactose or glucose. Because hemicellulose has a lower degree of polymerization and has a less regular structure than cellulose, it is more easily degraded by physical and chemical treatments.
Lignin is a hydrophobic macromolecular polymer with a complex structure. Lignin, in part, contributes to the protection of plants from various biochemical threats posed by insects and microorganisms such as mold. Because lignin is highly durable both physically and chemically, it is regarded as one of the most non-degradable compounds in nature.
In order to produce ethanol or various compounds from lignocellulose, polysaccharides forming the lignocellulose must be converted into fermentable sugars (sugar platforms) capable of ethanol fermentation. Liquid fuels such as ethanol and butanol, organic acids which are monomers of biopolymers such as polylactic acids, and various amino acids are producible from the fermentable sugars. The concept of the sugar platform was initially conceived by the U.S. Department of Energy (DOE). Here, conversion of lignocellulose into a sugar platform involves pretreating or fractionating the lignocellulose to produce sugars from cellulose and hemicellulose.
The pretreatment of the lignocellulose can be largely classified into physical, chemical, and biological methods.
Physical methods can include a milling process and a steam explosion process. The milling process includes crushing lignocellulose particles into small-size particles using a milling machine, thereby causing a structural change to the lignocellulose. The milling process is not frequently used because it consumes a considerable amount of energy and offers a low yield or a low saccharification. The steam explosion process includes steaming lignocellulose for a predetermined time in a high-pressure container of hot steam and opening a valve of the container instantaneously to allow the structure of the lignocellulose to be more accessible to enzymatic attack.
In order to increase the effects of the above-described physical methods, much research has been conducted on combinations of chemical and physical methods. A typical example of a combination of chemical and physical methods is a dilute-acid hydrolysis process. This process involves dipping lignocellulose in a 2% (w/w) or less solution of sulfuric acid and steaming the lignocellulose in a container of hot vapor for about 60 seconds to about 10 minutes at a temperature of about 160 to about 200° C. similarly to the steam explosion process. In this process, hemicellulose is hydrolyzed into monosaccharides and oligo-saccharides by acid catalysis and some pentose that is produced can be degraded into furfural by excessive reaction, which can act as a fermentation-inhibitor.
In the dilute-acid hydrolysis process, the hemicellulose is hydrolyzed to break bonds between the cellulose and the hemicellulose and between lignin and the hemicellulose, thereby facilitating enzymatic saccharification. Accordingly, a hemicellulose hydrolysate, such as xylose, which is hydrolyzed and dissolved in a hydrolyzate, can be obtained and separated during the fractionation process. Subsequently, both insoluble cellulose and lignin, which were not degraded during the fractionation process, are subjected to enzymatic saccharification, and then converted into glucose and lignin residues, so that the lignin can be transferred together to a subsequent fermentation process. In this case, however, phenolic compounds derived from the lignin degradation can inhibit the enzymatic process and the fermentation process.
An alternate method of fractionating a biomass using a base instead of an acid is the ammonia fiber explosion (AFEX) process developed by Bruce Dale et al. (“Enzyme hydrolysis and ethanol fermentation of liquid hot water and AFEX pretreated distillers' grains at high-solids loadings” Bioresource Technology, Volume 99, Issue 12, August 2008, Pages 5206-5215. Youngmi Kim, Rick Hendrickson, Nathan S. Mosier, Michael R. Ladisch, Bryan Bals, Venkatesh Balan, Bruce E. Dale). According to the AFEX process, ammonia and a biomass are mixed in a ratio of 1:1 to 1:3, the resulting mixture is treated at a high temperature for about 5 to about 30 minutes, and the pressure of a reaction vessel containing the mixture is explosively released to atmospheric pressure to retrieve gaseous ammonia and cause physical and chemical changes to the biomass structure, thereby improving the rate of enzymatic saccharification. In this process, little hemicellulose is hydrolyzed, but most lignin is dissolved and separated from cellulose and hemicellulose so that the cellulose and the hemicellulose can be saccharized during a subsequent enzymatic saccharification process to obtain glucose and pentose such as xylose.
A biological-based fractionation process can include pretreatment principally using mold, for example, white-rot fungus, which grow using saccharides obtained by degrading lignocellulose, under mild conditions. However, while the biological fractionation process is quite efficient, its productivity is relatively low and the enzyme is expensive so that it has not been put to large scale commercial and only used in small scale production.